The communication channel of a cellular mobile communication system is broadly classified into three channels, that is, a common channel, an individual channel, and a shared channel. The wireless base station transmits information as to the cell of the base station itself to all the mobile stations existing in the cell in a predetermined transmission power using the common channel. Further, a packet is transmitted in the individual channel in a power that varies as a time passes under a high speed closed loop type transmission power control so that the respective mobile stations satisfy predetermined communication quality. The shared channel is a channel for carrying out a packet communication by switching a plurality of mobile stations to a high speed. The transmission control of the shared channel is standardized as HSDPA by 3GPP, which is disclosed in, for example, Non-Patent Document 1 to be described later in detail.
FIG. 5 shows channels using the HSDPA between a wireless base station 100 and a mobile station 200. Although an example of FIG. 5 shows the one mobile station (200) for the convenience of explanation, it is assumed that even if a plurality of mobile stations exist, channels similar to those shown in FIG. 5 exist to the respective mobile stations. FIG. 5 shows a CPICH (Common Pilot Channel) 300 as a common channel, further shows an HS-PDSCH (High Speed Physical Downlink Shared Channel) 400A for transmitting data and an HS-SCCH (High Speed Shared Control Channel) 400B for transmitting control information as a shared channel used to HSDPA, and further shows, as an individual channel, an HS-DPCCH (High Speed Dedicated Physical Control Channel: individual control channel) 500 that is used by the mobile station to notify the wireless base station of a CQI (Channel Quality Indicator) which shows the communication path quality thereof.
According to the regulation of the HSDPA, the powers, which are obtained by subtracting the respective powers of the common channel and the individual channel from the maximum transmission power of the wireless base station 100, are allocated to HSDPA allocation powers which are the transmission powers allocated to the HS-PDSCH 400A and the HS-SCCH 400B. In the allocation, an average power in a predetermined period is used in place of the instant power of the individual channel in consideration of a high speed transmission power control in the individual channel. Further, to compensate the time-variation of the individual channel, the allocation is carried out after a transmission power margin is previously secured from the maximum transmission power. With this operation, it can be prevented that a power exceeding the maximum transmission power of the wireless base station 100 is allocated to the HSDPA allocation power.
A value, which is obtained by adding an offset to the power of a DL-DPCH (Download Dedicated Physical Channel: downlink individual channel), which is a pair with the HS-DPCCH 500 from the mobile station 200, is used as the transmission power of the HS-SCCH 400B for transmitting the control information in the shared channel. On the other hand, the transmission power of the HS-PDSCH 400A for transmitting data is determined by subtracting the transmission power of the HS-SCCH 400B from the allocation power of the HSDPA described above. However, the average value of an HS-SCCH power, which is a value obtained by adding an offset to the average value of the DL-DPCH, is ordinarily used as the power of the HS-SCCH 400B used at the time in consideration that the mobile station 200 in the cell is switched to a high speed.
Further, according to the regulation of the HSDPA, the wireless base station 100 selects a TFRC (Transport Format Resource Combination), which is a transmission format composed of a modulation type used to the HS-PDSCH 400A, the number of dispersion codes, and the number of information bits, using the HSDPA allocation power and the CQI reported from the mobile station 200 at each time at which a packet is transmitted and transmits data to the mobile station 200 based on the selected TFRC. The wireless base station 100 selects the TFRC using a correspondence table recommended in Non-Patent Document 1 and described below.
FIG. 6 shows an arrangement of the wireless base station 100 having a function for selecting the TFRC. FIG. 6 shows a part of a function in the wireless base station. In FIG. 6, a transmission candidate notification unit 1 notifies a TFRC allocation unit 4 of a mobile station to which data must be transmitted from the HSDPA. If a plurality of mobile stations exist as transmission candidates, the order in which TFRC is allocated to the respective mobile stations is also determined and notified at the same time. A power calculation unit 5 calculates an HS-PDSCH power 3 that can be used at the time and notifies the TFRC allocation unit 4 of it. A communication path quality notify unit 3 notifies the TFRC allocation unit 4 of the CQI obtained by the transmission candidate notification unit 1 from the mobile station. The TFRC allocation unit 4 selects the TFRC and allocates it to the respective mobile stations notified from the transmission candidate notification unit 1 in the notified order using the CQI and the HS-PDSCH power and notifies a data transmission unit (not shown) of the allocated TFRC.
A method of calculating the CQI, which is notified to the wireless base station by the mobile station, is disclosed in Non-Patent Document 1. That is, the mobile station measures the power of the CPICH 300 received from the wireless base station and newly predicts an HS-PDSCH power using the ratio of the measured power and the previously notified HS-PDSCH power. Further, the mobile station calculates an SIR (Signal to Interference Ratio) that is a receiving level when an HS-PDSCH packet is received based on the predicted HS-PDSCH power and searches a CQI which satisfies the previously determined SIR and has the largest number of transport bits from a previously prepared correspondence table of the TFRC and the CQI.
FIGS. 7 and 8 show an example of the correspondence table of the TFRC and the CQI. It is assumed that the correspondence table used by the mobile station is common to the correspondence table described above which is used by the wireless base station 100 to select the TFRC. FIGS. 7 and 8 are composed of the same items, respectively, and when, for example, FIG. 7 is referred to, a transport block size 7b, the number of HS-PDSCH codes 7c, a modulation type 7d, and a reference power adjustment value 7e are related to a CQI value 7a. Note that the correspondence table of FIG. 7 employs “QPSK” (Quadrature Phase Shift Keying: phase modulation) or “16-QAM” (16 Quadrature Amplitude Modulation: phase-amplitude modulation) as the modulation type, and the correspondence table of FIG. 8 employs only the “QPSK” as the modulation type. The mobile station reports the CQI value selected from the correspondence table as shown above to the wireless base station using the HS-DPCCH 500.
The wireless base station 100 selects the TFRC using the CQI from the mobile station as explained with reference to FIG. 6. However, since the HS-PDSCH power is varied by the variation of the number of individual channels, an error occurs between the HS-PDSCH power used by the mobile station to calculate the CQI and the HS-PDSCH power which can be used when the CQI reaches the wireless base station. To cope with this problem, the wireless base station corrects the error contained in the CQI from the mobile station using the ratio of the HS-PDSCH power previously notified to the mobile station and the HS-PDSCH power that can be allocated at the time and selects the TFRC from the correspondence table as shown in FIG. 7 using the corrected CQI.
Non-Patent Document 1: 3GPP, “3GPP TS25.214ver5.6.0 (2003-9)”, [online], September, 2003 (searched on Feb. 28, 2005) <http://www.3gpp.org/ftp/Specs/archive/25_series/25.214/25214-560.zip>